Respiration

Objectives

1. To describe oxidation and reduction in terms of electron and H+ transfer.

2. To distinguish anaerobic from aerobic cellular respiration in terms of ATP, oxygen, and chemiosmosis.

3. To demonstrate that carbon dioxide is a product of cell respiration.

4. To determine the effect of boiling on the aerobic respiration of bean seeds and explain the result in terms of enzyme activity.

 5. To measure the rate of oxygen consumption in germinating bean seeds.

6. To determine the metabolic rates for several small animals and relate this to body size and lifestyle. Introduction All organisms, whether plant or animal, bacteria, protests or fungi, carryout cellular respiration. During respiration organic food molecules are oxidized and these exergonic oxidation reactions are coupled with the synthesis of ATP, an endergonic reaction. The ATP is then used to drive the metabolic reactions necessary to maintain the organism’s physical integrity and to support all its other activities.

The cytoplasm of all cells contains the enzymes needed in the ancient central pathway of glycolysis, in which glucose is oxidized to pyruvate in the absence of oxygen. The energy released in this process is used to generate ATP directly by substrate level phosphorylation, in which phosphate groups are transferred directly from organic substrates to ADP.

To obtain energy from glucose, hydrogen atoms are removed from the glucose molecule as it is metabolized. These hydrogen atoms can be removed only by hydrogen (electron) carriers, such as NAD+. Cells contain a finite amount of NAD+ and since each NAD+ combines with only two hydrogens (electrons), there must be a mechanism for removing the hydrogens (electrons) from NADH so that glycolysis can continue.

In many organisms, respiration can occur under anaerobic conditions where no oxygen is present. Many bacteria, yeast, and animals ferment glucose, producing lactate or ethanol. During fermentation reactions, hydrogens are removed from glucose, passed to the electron carrier NAD+ (forming NADH), and then on to pyruvic acid (the end product of glycolysis), converting it to lactate or ethanol. Concurrently, the NADH is oxidized to NAD+, reconstituting the NAD+ pool required for glycolysis. Fermentation allows cells to make ATP in the absence of oxygen. Cells metabolizing glucose by fermentation harvest only about 5% of the available energy in glucose, however.

 Most organisms use molecular oxygen in a process called cellular respiration. In this series of reactions, the glucose molecule is completely disassembled to yield CO2 and H2O. The process begins with glycolysis; the end product of glycolysis, pyruvate, enters the mitochondrion where it is further metabolized.

After entering the mitochondrion, the pyruvate loses a CO2 molecule to form acetyl CoA, which enters a series of reactions known as the Kreb’s cycle or citric acid cycle where it is completely oxidized to CO2. The electrons released during this series of reactions are used to reduce NAD+, and a related molecule FAD, to NADH and FADH, respectively. These electron carriers then transfer their electrons to the electron transport chain (ETC), a series of proteins embedded in the inner membrane of the mitochondrion. The passge of electrons through the ETC generates an H+ gradient across the inner membrane which drives the synthesis of ATP by ATP synthetases embedded in the inner membrane. The last electron carrier protein in the ETC transfers the electrons to molecular oxygen to form water, with the addition of H+. This transfer of electrons to oxygen returns the protein (cytochrome oxidase) to its oxidized state so that it can continue to accept electrons from the remainder of the ETC.

The process of cellular respiration can be summarized by the following equation:

C6H12O6 +     O2 ->     6CO2 +    6H2O +   ATP

 Glucose    Oxygen   Carbon   Water    Energy

                             dioxide

 Complete respiration of one molecule of glucose results in a yield of 36-38 ATP molecules. Four of these ATP’s are the product of substrate-level phosphorylation; the remaining ATP’s are synthesized by the ATP synthetases using the energy of the H+ gradient created by electrons passing down the ETC in a process called oxidative phosphorylation. The coupling of electron flow with ATP synthesis is described in the chemiosmotic model.

 Overall, 18 times more ATP is produced by aerobic respiration than by anaerobic respiration. Even so, less than 50% (the actual number is about 39%) of the available energy in a glucose molecule is actually stored in ATP molecules. In contrast, fermentation and anaerobic respiration yield only two ATP’s. This is equal to only about 2% of the available energy in glucose. Clearly aerobic respiration gives a bigger energy payback than anerobic respiration.

In today’s lab you will be carrying out a series of experiments that will demonstrate several aspects of respiration including the release of carbon dioxide as a product of respiration, the uptake of oxygen by organisms during aerobic respiration, and attempt to demonstrate that metabolic rate (the rate of oxygen usage per gram of body weight) is a function of animal size.

Carbon Dioxide Production

Seeds contain stored food material in the form of some carbohydrate. When a seed germinates, the carbohydrate is broken down, liberating energy (ATP) needed for growth of the enclosed embryo into a seedling. Two days ago, a set of dry bean seeds was soaked in water to start the germination process. Another set was not soaked. This experiment will compare carbon dioxide production between germinating bean seeds, germinating bean seeds that have been boiled, and ungerminated (dry) bean seeds.

Procedure: 1. Obtain three respiration flask setups. One of these flasks will already contain ungerminated bean seeds and have the rubber stopper with attached fixtures already inserted.

 2. Fill one of the remaining flasks with germinated seeds to the same level as the flask of ungerminated seeds. Fill the remaining flask with boiled germinated seeds to the same level.

 3. Fit the rubber stoppers securely into the two flasks you filled. Add enough water to each test tube to cover the ends of the glass tubes coming out of the respiration flasks.

4. Set the flasks aside for about 1½ hours while you complete the remaining experiments.

5. After about 1½ hours, replace the water in each test tube with phenol red solution. Phenol red is a pH indicator, that is, it changes color in response to changes in pH. The stock solution is red (pH neutral), but in the presence of an acid the solution turns yellow. When CO2 is bubbled through water it forms a mild acid called carbonic acid

6. Pour water through the thistle funnel into each flask to force the gases through the glass tubing and into the phenol red solution.

 7. Record your results in table 1 and use these results to answer the questions below.

Table 1 CO2 Production

1. Which set of seeds was undergoing respiration?

2. What happened during boiling that caused the results you found? (Hint: Think about the effect of boiling on the enzymes)

Determining Metabolic Rate

The amount of oxygen used or carbon dioxide produced by the respiration of animals is one measure of all the organic processes going on within the animal. The term metabolism is sometimes used to describe this sum total of all life processes. There are many factors that influence the metabolic rate of an animal. Some of these are physical activity, diet, general state of health, and hormones such as those secreted by the thyroid gland and the adrenal cortex. The metabolic rate of an animal is also influence by whether the animal is warm-blooded or cold-blooded. Warm-blooded animals (homeotherms), which have the ability to maintain a relatively constant internal temperature, have a fairly uniform rate of metabolism. In the investigation here, you will measure the metabolism of several animals. Procedure:

NOTE: SODA LIME (NaOH) WILL HARM ANIMALS-DO NOT LET THE ANIMALS TOUCH THE SODA LIME!!!

 1. Prepare the container: Cover the bottom of the plastic chamber with a thin layer of soda lime (NaOH) (4 to 6 tablespoons full).

2. Weigh the empty wire cage.

3. Place the animal in the wire cage.

4. Weigh the animal in the cage.

5. Record the weight of the animal in table 3.

6. Place the animal (in the wire cage) into the plastic chamber.

7. Insert manometer apparatus into the open end of the plastic chamber.

8. Apply a drop of soap bubbles to the end of the manometer tube (shake the soap solution to make bubbles).

9. Determine the distance (in mm) that the soap bubble moves during three l-minute intervals. Record data in table 3.

10. Return the animal to the proper cage.

Calculate the respiratory rate of the animal by dividing the ml of O2 consumed by the time:

Respiratory rate = ml O2 consumed/time (min)

 The respiratory rate is expressed in ml O2 consumed/minute.

 Calculate the metabolic rate of the animal as follows:

Metabolic rate = Respiratory rate/weight of animal

Table 3. Animal respiration Rat Large mouse Small mouse Amphibian or Reptile

Answer the following questions.

1. What is the relationship between metabolic rate and the weight of the animal?

2. Use your data to compare the metabolic rate of a cold-blooded animal with that of a warm-blooded animal.